Catalytic system and process for the production of polydiolefins

ABSTRACT

A catalytic system to polymerize diolefinic monomers consists of a lanthanide salt, an organometallic compound of a metal belonging to groups I, II and III of the periodic table of elements and an organometallic compound of boron. The polymers obtained are characterized in that they have a high degree of 1,4 chain units and ratio between 1.4-cis/1,4-trans units which can vary as desired, and also a narrow molecular weight distribution.

This is a division of application Ser. No. 08/383,234 filed on Feb. 3,1995.

FIELD OF THE INVENTION

The present invention relates to a catalytic system obtained by theinteraction of a salt of a metal belonging to the group of lanthanides,an aluminium alkyl and a trialkylic derivative of boron. The presentinvention also relates to the preparation of said catalytic system andits use in the polymerization of diolefins, wherein the content of1,4-cis and 1,4-trans units of the polydiolefins obtained can vary byacting on the same catalyst.

The polydienes produced with this catalytic system are characterized inthat they have a narrow molecular weight distribution [expressed as aratio between the weight average molecular weight (M_(w)) and the numberaverage molecular weight (M_(n))].

A metal belonging to the group of lanthanides, as is generally acceptedin the known art, means a metal belonging to the group comprising:Scandium, having an atomic number of 21; Yttrium, having an atomicnumber of 39; a metal having an atomic number between than of Lanthanum(57) and Lutetium (71). These metals are part of group IIIA of theperiodical table, according to the IUPAC definition prior to 1985.

In addition for some of the formulae indicated in the text the followingacronyms are used: TMA=Al(CH₃)₃ ; TEA=Al(C₂ H₅)₃ ; TIBA=Al(C₄ H^(i) ₉)₃; DIBAH=AlH(C₄ H^(i) ₉)₂ ; Nd(Ver)₃ =neodymium versatate; BPFF=B(C₆F₅)₃.

DESCRIPTION OF THE BACKGROUND

In the known art, there are ample descriptions of catalytic systems withthree components, based on derivatives of lanthanides (rare-earth), forthe polymerization of butadiene having a polymer with a high 1,4-cischain units.

For example German patents 1812935, 2011543, 2833721, 2830080, Chinesepatent 85101199 and International PCT patent 93-05083, describe thepreparation of polybutadiene with a high degree of 1,4-cis with the useof catalytic systems based on compounds of rare-earth and aluminiumtrialkyls.

In all cases the presence of a halogenating agent is essential,generally a derivative of boron having the general formula BRn_(n)X_(3-n), of aluminium having the formula AlR_(n) X_(3-n), silicon havingthe general formula SiR_(n) Cl_(4-n), wherein R is an alkyl radical andX is a halogen atom such as Cl, Br or I.

The type of organometallic compound of aluminium or halogen Used caninfluence the activity of the catalyst and the molecular weight of thefinal polymer but the stereospecificity is not influenced in any way.

The examples provided in the patents of the known art mentioned aboveindicate that the catalytic systems can be prepared both with thewell-known technique "in situ" or preformed, with or without an agingperiod, before their use in the polymerization reaction.

Interesting examples of preformed catalytic systems with an easyindustrial preparation and having a high activity are those described inEuropean patents 201962, 201979 and 207559, wherein an alkyl halide, forexample t-Butylchloride, is used as halogenating agent.

All of the documents of the known art quoted so far clearly demonstratethe production of polymers with a high degree of 1,4-cis units, startingfrom preformed catalytic systems, or systems prepared "in situ",comprising an aluminium trialkyl, a lanthanide salt and a halogenatingagent.

The known art also describes the use of binary catalytic systems basedon lanthanide salts and aluminium trialkyls. In the latter case thestereospecificity of the final polymer greatly depends on the type oflanthanide salt. In this way, by using lanthanide halides as such or inthe form of complexes with alcohols, amines, organic acids oralkylphosphates, together with aluminium trialkyls, polydiolefins areobtained, which, in the case of butadiene, have chains of monomericunits of the 1,4-cis type of over 90% (typically 95-98%).

Valid examples of catalytic systems of this type are disclosed inJapanese patents 8361107 and 84113003, U.S. Pat. No. 4,575,538, Germanpatent 243034, Chinese patent 1036962 and Russian patent 675866respectively.

When the binary system is obtained starting from an aluminium trialkyltogether with an oxygenated lanthanide salt, with the rigorous exclusionof any form of halogenating agent, either organic or inorganic, thefinal polymer is a polybutadiene with a high content (74-90%) of1,4-trans units. Examples of these catalytic systems are claimed inpatents EP 091287 and JP 9060907, which describe the use of binarysystems based on a lanthanide carboxylate and an alkyl derivative ofmagnesium (MgBut₂) or lithium (Bu--Li) together with an aluminiumtrialkyl (AlEt₃) respectively. In both cases, apart from the use ofcostly components such as the alkyls of lithium or magnesium, thecatalyst has a limited activity and requires long polymerization times(24 hrs) to reach industrially acceptable conversions. In addition, onlyplastomers with a high 1,4-trans degree can be obtained without thepossibility of changing the stereospecificity of the polymer to highervalues of 1,4-cis units typical of an elastomer.

The documents of the known art mentioned above clearly indicate thatelastomeric polybutadienes with a high degree of 1,4-cis units can onlybe obtained with ternary or binary systems when halogen atoms in theform of organic or inorganic halogenating agents are present in thecatalytic mixture, or starting from halogenated salts of lanthanides.Without these, only polymers with a high degree of 1,4-trans units(>75%) can be obtained and it is not possible to vary the relativepercentage value of 1,4-Trans and 1,4-Cis units.

SUMMARY OF THE INVENTION

Research carried out by the Applicant has, on the other hand,surprisingly discovered a new ternary catalytic system, based onlanthanides, capable of supplying an elastomeric polybutadiene withvarying values of 1,4-Cis and 1,4-Trans units as required withoutorganic or inorganic halogenating agents, such as those indicated in theabove documents; more specifically the new ternary catalytic systemconsists of:

(a) a lanthanide salt having the general formula ML₃ ;

(b) an aluminium alkyl having the general formula AlR¹ ₃ ;

(c) a derivative of boron having the general formula BR² _(3-m) (C₆H_(5-n) R³ _(n))_(m), the meaning of M, L, R¹, R² and R³ being explainedin detail below.

The catalytic system is formed when the three components are reacted insuitable molar ratios and under suitable experimental conditions, aswill be better illustrated hereafter.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with what is specified above, a first aspect of thepresent invention relates to a catalytic system for the polymerizationof dienic monomers, basically consisting of:

(A) a salt having the general formula

    ML.sub.3

wherein M represents a metal such as Sc, Y or a metal with an atomicnumber of between 57 and 71 and L a general anionic binder with the soleexclusion of halogen binders,

(B) an alkyl, hydride or alkyl-hydride compound having general formula

    MeR.sup.1.sub.z

wherein,

Me is a metal belonging to groups I, II or III of the periodic table;

R¹ is:

an aliphatic or cycloaliphatic radical containing from 1 to 20 carbonatoms,

an aromatic radical containing from 6 to 20 carbon atoms,

a hydrogen atom,

z is an integer from 1 to 3 equal to the value of Me, on condition that,if z is equal to 3, only one of the three R¹ groups bound to the metalcan be hydrogen, (C) an organometallic derivative of boron havinggeneral formula

    BR.sup.2.sub.3-m (C.sub.6 H.sub.5-n R.sup.3.sub.n).sub.m

wherein,

R² is an aliphatic radical, linear or branched, containing from 1 to 20carbon atoms; a cycloaliphatic group containing from 6 to 20 carbonatoms; an aromatic group containing from 6 to 20 carbon atoms,

R³ is a fluorine atom or CF₃ group;

m is an integer between 1 and 3

n is an integer between 1 and 5.

As mentioned briefly above, component (A) of the catalytic system,having general formula ML₃, consists of an Sc, Y or lanthanide salt withan atomic number of between 57 and 71.

Although the catalytic system claimed is absolutely general, for variousreasons, with respect to availability, commercial price and catalyticactivity, as well as the characteristics of the final polymer, the saltshaving general formula ML₃ where M is Nd, Pr, Dy, La, Gd and Y arepreferred.

The binder L is a general anionic binder with the only condition that itis not halogen. Bearing in mind what is specified above, valid,non-restrictive examples of salts of the metals claimed above arecarboxylates such as naphthenate, versatate, pivalate, 2-ethylhexanoate,formiate, acetate, trifluoroacetate; alkoxides such as methylate,butylate, ter-butylate; phenolates; thioalkoxides; dialkylamides;bis-trimethylsilylamides; acetylacetonates andhexafluoroacetylacetonates.

Component (B) of the catalytic system is represented by a hydride, alkylor mixed derivative of a metal belonging to groups I, II or III of theperiodic table of elements. Valid but non-restrictive examples of thiscompound can be: lithium hydride, lithium-aluminium hydride, lithiumbutyl, lithium sec-butyl, sodium hydride, magnesium hydride, magnesiumdibutyl, aluminium trimethyl, aluminium triethyl, aluminium triisobutyl,aluminium diisobutyl-monohydride, aluminium trioctyl, gallium trimethyl,gallium triethyl. For reasons of solubility, industrial availability andprice, the alkyl derivatives of aluminium such as aluminium trimethyl(TMA), aluminium triethyl (TEA), aluminium triisobutyl (TIBA) andaluminium diisobutylmonohydride (DIBAH) are preferred.

Component (C) of the catalytic system consists of an organometallicderivative of boron having the general formula

    BR.sup.2.sub.3-m (C.sub.6 H.sub.5-n R.sup.3.sub.n).sub.m

wherein

R² is an aliphatic radical, linear or branched containing from 1 to 20carbon atoms; a cycloaliphatic group containing from 6 to 20 carbonatoms; an aromatic group containing from 6 to 20 carbon atoms;

R³ is a fluorine atom or CF₃ group;

m is an integer between 1 and 3;

n is an integer between 1 and 5.

Non-limiting examples of this group of derivatives are: B(C₆ F₅)₃,B(CH₃) (C₆ F₅)₂, B(C₂ H₅) (C₆ F₅)₂, B(C₆ H₄ F)₃, B(C₆ H₃ F₂)₃, B(C₆ H₂F₃)₃, B[C₆ H₃ (CF₃)₂ ]₃, B[C₆ H₂ (CF₃)₃ ]₃, B(C₂ H₅)[C₆ H₃ (CF₃)₂ ]₂.

As specified above, the catalytic system of the present invention isprepared by mixing in suitable ratios components (A), (B) and (C),previously described, in an aliphatic, cycloaliphatic or aromaticsolvent or their mixtures. This preparation can be carried out eitherwith "in situ" or with preformation techniques.

In the former case, the preparation of the catalytic system is carriedout by adding, in order, component (B), the monomer to be polymerized,component (A), and, last of all, component (C), to the solvent,obtaining a limpid solution and the polymerization is carried out in ahomogeneous phase. The order of introducing the monomer, component (A)and component (B) is not binding whereas, as far as component (C) isconcerned, the best results are obtained by adding this as last.

In the latter case, the preformation of the catalytic system is carriedout by reacting components (A), (B) and (C), in the order mentionedabove, in the desired solvent, for a period of 0.5-24 hours attemperatures of between 0° and 80° C. in the presence of or withoutsmall quantities of the diolefin to be polymerized. Aliphatic,cycloaliphatic or aromatic hydrocarbons can be used in the preformationreaction. The use of an aromatic solvent gives a homogeneous solution,whereas the use of an aliphatic solvent produces a partial formation ofsolid precipitate. The formation of the precipitate however does notnegatively influence the catalytic activity as the precipitate formedduring the preformation of the catalyst redissolves in thepolymerization environment forming a homogeneous phase.

Experimental work has shown that the best preformation conditions areobtained by carrying out the reaction between (A), (B) and (C) intoluol, in the order specified above, at 50° C. for 1 hour in thepresence of or without less than 1 g of butadiene per 1×10⁻³ moles ofcomponent (A).

In the formation of the catalytic system, the molar ratios in whichcomponents (A), (B) and (C) are reacted, are of considerable importance.Research carried out by the applicant has shown that the molar ratio(B)/(A) can vary between 3 and 100 and, preferably, between 8 and 20.Values higher than 100, although they can be used, are not advisable asthey do not provide any improvements in the catalytic process and thecost increases due to component (B). The molar ratio (C)/(A) can varybetween 0.1 and 50 but values between 1 and 3 are preferred.

As already mentioned, a further aspect of the present invention relatesto the use of the catalytic system described above in a polymerizationprocess of conjugated diolefins, (for example butadiene), this processbeing characterized by a high polymerization rate and by the production,with high yields, of a polybutadiene with a varying degree of 1,4-cisand 1,4-trans units, controllable molecular weights and a narrowmolecular weight distribution. More specifically, the resulting polymerof this process has percentage values of 1,4-cis and 1,4-trans unitswhich vary from 35/63 to 98/1 respectively, whereas the percentage valueof 1,2 units is generally between 0.5 and 2. In addition, the value ofthe weight average molecular weight (Mw) can vary between 50×10³ and1×10⁸ and the ratio between the weight average molecular weight (Mw) andthe number average molecular weight (Mn) is between 1.5 and 2.6.

It can be easily confirmed that all the alkyl derivatives generate,together with compounds (A) and (C), a highly active catalytic system inthe polymerization of high polymer diolefins with a 1,4 chain units ofmore than 98%. Research carried out by the Applicant has shown howeverthat the nature of component (B) has a determining influence on thecomposition of the final polymer. Consequently the use of TMA produces afinal polymer having a percentage of 1,4-cis/1,4-trans units of about30/70 respectively whereas the use of TIBA produces a polymer with a98/1 ratio of the same units. The catalysts prepared with TEA and DIBAHproduce polymers having intermediate ratio values between the 1,4-cisand 1,4-trans units.

Other differences have been verified depending on the nature of compound(B) and relating to the activity of the catalytic system and value offinal molecular weights of the polymer. It is well-known to experts inthe field, however, that in a Ziegler-Natta type polymerization, thesecharacteristics often depend on the nature of the alkyl derivatives usedfor the preparation of the catalytic system and there are plenty ofexamples in the known art in this respect.

Even if the catalytic system claimed by the applicant is active in thepolymerization of unsaturated hydrocarbon derivatives, in particular,diolefins such as 1,3-butadiene, isoprene, 1,3-pentadiene,2,3-dimethylbutadiene, the monomer preferably used is 1,3-butadiene asthe polymers of this monomer are products which are the most widely usedindustrially.

The activity of the catalytic system depends on the nature of components(A) and (B) used and on the formation method of the catalytic system.Alkoxide derivatives therefore prove to be generally more active thancarboxylate derivatives and TIBA has a similar activity to DIBAH andboth are better than TMA. In general, with all the aluminium alkyls, thebest activity has been obtained by preforming the catalytic system inthe presence of or without small quantities of monomer and aging for 1hour at a temperature of 50° C. A particularly interesting fact is thatthe activity of preformed catalytic systems, in the presence of orwithout monomer, remains high even after conservation periods of thecatalytic system of several weeks.

The polymerization reaction is carried out indifferently in an aliphaticsolvent, n-hexane or a C₆ petroleum cutting with a boiling point ofbetween 64° and 68° C., or in an aromatic solvent, toluene. Aninteresting characteristic of the catalytic system claimed in thepresent invention is its high activity in polymerization with the doubleadvantage of minimizing the cost of the catalytic system and thequantity of impurities which remain in the polymer after its recovery.In fact, activities equal to 5-10×10⁻⁴ moles of component (A) per Kg ofdiolefin polymerized are obtained with reaction times of between 0.5 and2 hours at temperatures of between 0° and 100° C.

A further aspect of the present invention relates to the polymerobtained using the above catalytic system. It consists of polybutadienewith a high 1,4 chain units content in which it is possible to vary theratio between the 1,4-cis and 1,4-trans type units from 30/70 to 98/1depending on the type of component (B) used. These variable cis-transpolymers form an interesting class of materials ranging from elastomerswhich are crystallizable under stretch, those with 1,4-Cis units >95%,to completely amorphous elastomers at room temperature, with 50% in1,4-trans units.

A common characteristic of the whole group of polymers obtained with thecatalytic system claimed, and this is a further aspect of the presentinvention, is the low value of molecular weight distribution of thepolymers intended as a ratio between the weight average molecular weightand the number average molecular weight (Mw/Mn), of between 1.5 and 2.6.

This narrow distribution ensures that there are no high or low molecularweight fringes in the polymer which are always present when the Mw/Mndistribution exceeds the value of 3. In this respect, the polymerizationcarried out at 0° C. is to be particularly interesting, as in this casehigh molecular weights and a molecular weight distribution (Mw/Mn) of1.6 are obtained.

Another interesting characteristic of the polymers produced according tothe present invention is the lack of any type of gel (micro or macrogel) which is often present in polybutadiene with a high 1,4-Cis chainunits with a weight average molecular weight higher than 500×10³.

EXAMPLES

Following this general description of the catalytic system andpolymerization process of the present invention, detailed examples ofthe preparation of the catalyst, polymerization reaction and polymerobtained, are given below. These examples, which are not limiting,describe the experimental operations in order to provide a betterunderstanding of the invention itself.

Example 1

Preparation of the preformed catalyst.

Procedure A. 20 cm³ of toluene, 1.65×10⁻³ moles of component (A) and29.7×10⁻³ moles of component (B) are introduced in an inert atmosphereinto a 100 cm³ graded test-tube, equipped with a lateral tap for thenitrogen flow and a magnetic anchor. The mixture is left under stirringuntil the solid has completely dissolved and 4.95×10⁻³ moles ofcomponent (C) are then added. Using the graded scale of the test-tube,the final volume is brought to 50 cm³ with toluene and the mixture isleft to age at room temperature under stirring for the desired time. Thesolution of preformed catalyst thus obtained, which contains 3×10⁻³gAtoms/cm³ of metal of component (A), is used for the polymerizationtests.

Procedure B. Following the procedure of process A, a toluene solution ofcatalyst is prepared by introducing, in order, 0.9×10⁻³ moles ofcomponent (A), 16.2×10⁻³ moles of component (B) and 2.7×10⁻³ moles ofcomponent (C). The solution is brought to 30 cm³ with toluene and ismaintained at 50° C. for 1 h, under stirring. The concentration provesto be 3×10⁻³ gAtoms/cm³ of metal of component (A).

Procedure C. 20 cm³ of toluene, 1.15×10⁻³ moles of component (A) and20.7×10⁻³ moles of component (B) are introduced in an inert atmosphereinto a 100 cm³ graded test-tube, equipped with a lateral tap for thenitrogen flow and a magnetic anchor. The mixture is left under stirringuntil the solid has completely dissolved and 1.2 g of liquid butadieneand 3.4×10⁻³ moles of component (C) are then added, in order. Using thegraded scale of the test-tube, the final volume is brought to 38 cm³with toluene and the preformation reaction is carried out under stirringand at room temperature for the desired time. The final solution ofpreformed catalyst thus obtained, which contains 3×10⁻³ gAtoms/cm³ ofmetal of component (A), is used for the polymerization tests.

Procedure D. Following the procedure of process A, a toluene solution ofcatalyst is prepared by introducing, in order, 25 cm³ of toluene,1.3×10⁻³ moles of component (A), 23.4×10⁻³ moles of component (B), 1.4 gof liquid butadiene and 3.9×10⁻³ moles of component (C). The solution isbrought to 43 cm³ with toluene and is maintained at 50° C. for 1 h,under stirring. The concentration proves to be 3×10⁻³ gAtoms/cm³ ofmetal of component (A).

Example 2

The polymerization of butadiene is described using a catalyst preparedin the presence of a monomer and without aging ("in situ" preparation).

A 200 cm³ drinking bottle, previously heated in a muffle furnace at 140°C., is cooled to room temperature under a perfectly dry nitrogen flow,to eliminate the environmental air and humidity. The following productsare charged into the bottle which is maintained under a nitrogenatmosphere and immersed in a bath at 0° C.: 150 cm³ of anhydrous hexane,20 g of anhydrous butadiene liquid removed from an overturned cylinderwith a hypodermic syringe sealed to the cylinder valve, 1 cm³ of ahexane solution containing 3×10⁻⁵ moles/cm³ of Nd(Obu)₃, 0.54 cm³ of a 1molar solution of Al(C₄ H₉ ^(i))₃ and 0.18 cm³ of an 0.5 molar solutionof B(C₆ F₅)₃. The bottle is then closed with a crowned top equipped witha teflon seal, placed in a bath at 50° C. and magnetically stirred for 4hrs. The bottle is then rapidly cooled in a bath at 0° C., opened andthe contents poured in about 300 cm³ of methanol containing 0.5 g of asuitable antioxidant. The coagulated polymer is collected, washed threetimes with methanol and then dried in a vacuum oven heated to 60° C.obtaining 19 g (95% conversion) of dry polymer whose structure, upon IRanalysis, proves to be 92% 1,4-cis, 7% 1,4-trans, 1.0% 1,2. GelPermeation Chromatography analysis shows that the polymer has a Mw of200×10³ and Mw/Mn=2.

Examples 3-6

The influence of the type of component (B) on the stereospecificity ofthe polymer obtained is shown. Following the operating procedure, theconcentrations and molar ratios indicated in example 2, four catalystsare prepared with the "in situ" technique, using Nd(OBu)₃ as component(A) and BPFF as component (C). The type of component (B) used, thepolymerization conditions and results obtained are shown in table 1.

Example 7

Butadiene is polymerized using a preformed catalyst.

A 200 cm³ drinking bottle, previously heated in a muffle furnace at 140°C., is cooled to room temperature under a perfectly dry nitrogen flow,to eliminate the environmental air and humidity. The following productsare charged into the bottle which is maintained under a nitrogenatmosphere and immersed in a bath at 0° C.: 150 cm³ of anhydrous hexaneand 20 g of anhydrous butadiene. To this solution, 1 cm³ is added of asolution containing 3×10⁻³ gAtom of neodymium, preformed catalystprepared with the procedure indicated in example 1 Procedure A using1.1×10⁻³ moles of Nd(Ver)₃, 19.8×10⁻³ moles of TIBA and 3.3×10⁻³ molesof BPFF and preforming at room temperature for 24 hrs. The bottle isthen closed with a crowned top equipped with a teflon seal, placed in abath at 50° C. and magnetically stirred for 1 h. After this period, thebottle is then rapidly cooled in a bath at 0° C., opened and thecontents poured in about 300 cm³ of methanol containing 0.5 g of asuitable antioxidant. The coagulated polymer is collected, washed threetimes with methanol and then dried in a vacuum oven heated to 60° C.obtaining 18.6 g (93% conversion) of dry polymer whose structure, uponIR analysis, proves to be 96% 1,4-cis, 3% 1,4-trans, 1.0% 1,2. GelPermeation Chromatography analysis shows that the polymer has a Mw of482×10³ and Mw/Mn=2.

Example 8

Following the operating procedure described in example 2, a catalyst isprepared "in situ" by charging into a drinking bottle 150 cm³ of hexane,24 g of butadiene, 1 cm³ of a hexane solution containing 7×10⁻³moles/cm³ of Nd(Ver)₃, 1.3 cm³ of a 1 molar solution of AlH(C₄ H₉ ^(i))₂and 0.42 cm³ of an 0.5 molar solution of B(C₆ F₅)₃. The polymerizationis carried out for 4 hrs at 50° C. obtaining 18 g (75%) of polymer withthe following analyses: 64% 1,4-cis; 34% 1,4-trans; 2% 1,2; Mw=200×10³ ;Mw/Mn=2.6.

Example 9

A preformed catalyst is prepared according to the procedure described inexample 1 Procedure A using as components of the catalytic system0.7×10⁻³ moles of Nd(Ver)₃, 12.6×10⁻³ moles of TMA, 2.1×10⁻³ moles ofBPFF and preforming for 1 hr at 50° C. Following the method indicated inexample 7, 1 cm³ of the catalyst solution previously prepared is addedto 20 g of butadiene in 150 cm³ of hexane, the polymerization beingcarried out at 50° C. for 1 hr. 9.0 g (45%) of dry polymer are obtained,whose structure, upon IR analysis, proves to be 32% 1,4-cis, 67%1,4-trans, 1.0% 1,2. Gel Permeation Chromatography analysis shows thatthe polymer has a Mw of 332×10³ and Mw/Mn=2.4.

Example 10

Following the operating procedure described in example 2, a catalyst isprepared "in situ" by charging into a drinking bottle 150 cm³ of hexane,20 g of butadiene, 1 cm³ of a hexane solution containing 3×10⁻³moles/cm³ of Pr(OBu)₃, 0.54 cm³ of a 1 molar solution of Al(CH₃)₃ and0.18 cm³ of an 0.5 molar solution of B(C₆ F₅)₃. The polymerization iscarried out for 4 hrs at 50° C. 14.4 g (72%) of polymer are recovered.

Examples 11-17

Three preformed catalysts are prepared by reacting, according to theprocedure and ratios indicated in example 1 Procedures A and B, Nd(OBu)₃or Pr(OBu)₃, as components (A), with aluminium trialkyl, as component(B), and BPFF, as component (C). The polymerization tests carried out asin example 7, show how the preformation technique considerably increasesthe catalyst activity and this activity remains even for longpreformation periods. Components (A) and (B) used, the temperatures andpreformation times, the yields and characteristics of the polymers areshown in table 2.

Examples 18-22

Four preformed catalysts are prepared starting from Nd(OBu)₃ ascomponent (A), TIBA as component (B) and BPFF as component (C). Thepreformation is carried out according to the procedure described inexample 1 Process C and D. The polymerization reactions were carried outas illustrated in example 7 and show how preformation in the presence ofsmall quantities of monomer to be polymerized considerably increases thecatalyst activity and this activity remains even for long preformationperiods without great variations in the characteristics of the polymer.The temperatures and preformation times, the polymerization conditionsand results obtained are shown in table 3.

Examples 23-28

Two preformed catalytic systems are prepared starting from Nd(Ver)₃ ascomponent (A), TMA and TIBA as components (B) and BPFF as component (C).The catalytic systems are preformed, according to the procedureindicated in example 1 Procedures C and D, at room temperature for 24hrs. The polymerization tests, carried out as indicated in example 7,show the influence of the polymerization temperature on the molecularweights of the polymers produced. The polymerization temperatures andresults obtained are shown in table 4.

                  TABLE 1    ______________________________________                   Con-    Ex.* Component version Infrared Analysis (%)                                        Mw    Mw/    nr.  (B)       %       1,4 cis                                 1,4 trans                                        1,2 × 10.sup.-3                                                  Mn    ______________________________________    3    TMA       98      92     7     1   200   2.0    4    "         93      80    19     1   322   2.6    5    "         85      59    40     1   130   2.6    6    TIBA      86      42    57     1   429   2.4    ______________________________________     (*)Solvent hexane cm.sup.3 150; Nd (OBu).sub.3 3 × 10.sup.-5 moles;     Component (B) 5.4 × 10.sup.-4 moles; BPFF 9 × 10.sup.-5 moles     Butadiene g 20; Polymerization temperature 50° C.; Time 4 hrs.

                  TABLE 2    ______________________________________            Component   Preformation  Yield    Example* nr.              (A)       (B)     Time (hrs.)                                        T (°C.)                                              %    ______________________________________     11**     Nd (OBu).sub.3                        TIBA    --      --    35    12        "         "        1      room  46    13        "         "       24      room  95    14        "         "       168     room  93    15        "         TMA     24      room  84    16        Pr (OBu).sub.3                        TIBA     1      50    80    17        Nd (OBu).sub.3                        "        1      50    97    ______________________________________     (*)Solvent hexane cm.sup.3 150; 1 cm.sup.3 of solution of preformed     catalyst equal to 3 × 10.sup.-5 gAtom of Lanthanide; Butadiene g 20     Polymerization temperature 50° C.; Polymerization time 1 hr.     (**)Test "in situ" as in example 2.

                                      TABLE 3    __________________________________________________________________________    Ex.*       Preformation             Polimerization                      Infrared Analysis (%)                                 Mw    nr.       t (h)          T (°C.)              T (°C.)                  Yield %                      1,4 cis                          1,4 trans                               1,2                                 × 10.sup.-3                                     Mw/Mn    __________________________________________________________________________     18**       -- --  50  35  90  9    1 120 2.4    19  1 room              "   60  94  5    1 250 2.1    20 24 "   "   80  92  7    1 390 2.2    21 120          "   "   93  92  7    1 450 2.2    22  1 50  "   80  nd  nd   nd                                 nd  nd    __________________________________________________________________________     (*)Solvent hexane cm.sup.3 150; 1 cm.sup.3 of solution of preformed     catalyst equal to 3 × 10.sup.-5 gAtom of Neodimium; Butadiene g 20;     Polymerization time 1 hr.     (**)Test "in situ" as in example 11.

                                      TABLE 4    __________________________________________________________________________    Ex.*       Component             Polimerization                         Infrared Analysis (%)                                    Mw    nr.       (B)   T (°C.)                 t (hrs)                     Yield %                         1,4 cis                             1,4 trans                                  1,2                                    × 10.sup.-3                                        Mw/Mn    __________________________________________________________________________    23 TMA    0  15  70  37  62   1 700 1.8    24 "     30  3   90  39  60   1 490 1.7    25 "     50  1   84  40  59   1 350 2.1    26 TIBA   0  3   95  98   1   1 750 1.6    27 "     30  1   98  92   7   1 509 1.7    28 "     80  1   90  77  21   1 300 2.6    __________________________________________________________________________     (*)Solvent hexane cm.sup.3 150; 1 cm.sup.3 of solution of preformed     catalyst equal to 3 × 10.sup.-5 gAtom of Neodimium; Butadiene g 20.

We claim:
 1. A method of polymerizing diolefinic monomers, comprisingcontacting diolefin monomers with a catalytic system consistingessentially of:(A) a salt having the formula

    ML.sub.3

wherein M represents Sc, Y or a metal with an atomic number of 5 to 71and L is an anionic ligand with the sole exclusion of halogen, (B) analkyl, hydride or alkyl-hydride compound having the formula

    MeR.sup.1.sub.z

wherein, Me is a metal of group I, II or III of the periodic table; R¹is: an aliphatic or cycloaliphatic radical containing from 1 to 20carbon atoms, an aromatic radical containing from 6 to 20 carbon atoms,or a hydrogen atom,z is an integer from 1 to 3 equal to the valence ofMe, on condition that if z is equal to 3, only one of the three R¹groups bound to the metal is hydrogen, (C) an organometallic derivativeof boron having the formula

    BR.sup.2.sub.3-m (C.sub.6 H.sub.5-n R.sup.3.sub.n).sub.m

wherein R² is an aliphatic radical, linear or branched, containing from1 to 20 carbon atoms; a cycloaliphatic group containing from 60 to 20carbon atoms; or an aromatic group containing from 6 to 20 carbon atoms,R³ is a fluorine atom or CF₃ group; m is an integer from 1 to 3; and nis an integer from 1 to
 5. 2. The method of claim 1, wherein saiddiolefinic monomers are selected from the group consisting of1,3-butadiene, isoprene, 1,3-pentadiene, 2,3-dimethylbutadiene andmixtures thereof.
 3. The method of claim 1, wherein a polymer producedby said method has a ratio of 1,4-cis/1,4-trans units of from 35/63 to98/1.
 4. The method of claim 1, wherein M represents Nd, Pr, Dy, La, Gdor Y.
 5. The method of claim 1, wherein L is a carboxylate.
 6. Themethod of claim 1, wherein L is an alkoxide.
 7. The method of claim 1,wherein L is selected from the group consisting of a phenolate, athioalkoxide, a dialkylamide, a bistrimethylsilyl-amide, an acetylacetonate and a hexafluoroacetyl acetonate.
 8. The method of claim 1,wherein (B) said alkyl, hydride, or alkyl-hydride compound having theformula MeR¹ _(z) is selected from the group consisting of lithiumhydride, butyl lithium, sec-butyl lithium, sodium hydride, magnesiumhydride, dibutyl magnesium, trimethyl aluminum, triethyl aluminum,triisobutyl aluminum, diisobutyl aluminum-monohydride, trioctylaluminum, trimethyl gallium and triethyl gallium.
 9. The method of claim1, wherein (B) said alkyl, hydride, or alkyl-hydride compound having theformula MeR¹ _(z) is selected from the group consisting of trimethylaluminum, triethyl aluminum, triisobutyl aluminum and diisobutylaluminum-monohydride.
 10. The method of claim 1, wherein L is selectedfrom the group consisting of naphthenate, versatate, pivalate,2-ethyl-hexanoate, formiate, acetate or trifluoroacetate.
 11. The methodof claim 1, wherein L is selected from the group consisting of amethylate, butylate, tert-butylate or isopropylate.